1. Field of the Invention
The present invention relates generally to self-assembled semiconductor quantum dot devices. More particularly, the present invention is directed towards self-assembled quantum dot devices for use in opto-electronic devices.
2. Description of Background Art
Quantum dot and quantum wire semiconductors structures are of interest for a variety of electronic and opto-electronic device applications. A semiconductor quantum dot is a structure having energy barriers that provide quantum confinement of electrons and holes in three dimensions. A semiconductor quantum wire is a structure having energy barriers that provide quantum confinement of electrons and holes along two dimensions.
Theoretical studies indicate that quantum dot and quantum wire lasers have many potential performance advantages over conventional quantum well lasers. First, a quantum dot or quantum wire laser has a lower fill factor (volume of material to be pumped) and an improved density of states function compared with a quantum well laser. Referring to FIG. 1, the theoretical density of states function become sharper as the carrier dimensionality decreases. FIG. 1A shows a theoretical density of states function for a bulk material, which has a square root dependence on energy. FIG. 1B shows the theoretical density of states function for a quantum well (one dimension of quantum confinement) which increases in steps at each quantum well energy level. FIG. 1C shows the theoretical density of states function for a quantum wire (two dimensions of quantum confinement). FIG. 1D shows the theoretical density of states function for a quantum dot (three dimensions of quantum confinement) which has a delta-like density of states function (e.g., a finite number of states available only at the quantum dot). Theoretical calculations indicate that the threshold current of a semiconductor laser may be improved by using quantum dot active regions due to the smaller volume of material and reduced number of states.
Quantum wire and quantum dot lasers emitting light in the 1.3 to 1.6 micron wavelength range are of particular interest for fiber optic communication systems. Commonly, InGaAsP is used to fabricate long wavelength quantum well lasers. Conventional approaches to fabricating quantum dots in InGaAsP include etching and regrowth techniques to pattern and embed InGaAsP quantum dots. Unfortunately, the drawbacks of conventional approaches to fabricating quantum dot and quantum wire lasers have limited the commercial applications of quantum dots, particularly in the 1.3 to 1.6 micron emission wavelength range. One potential problem in fabricating quantum dot and quantum wire lasers is deleterious non-radiative interface recombination. Quantum dots and quantum wires have a large surface-to-volume ratio which render them especially sensitive to interface defects. Additionally, the small fill factor of quantum dot and quantum wire active regions can cause significant current leakage, i.e., a significant fraction of the laser drive current is not injected into the quantum dots or quantum wires and/or is depleted from the quantum structure by thermionic emission.
The drawbacks of conventional quantum dot laser fabrication methods can result in a threshold current that is much greater than the theoretical limit. Additionally, these same drawbacks can make it difficult to form semiconductor active regions capable of lasing over a wide wavelength range. In conventional quantum well lasers, the peak of the gain spectrum shifts to shorter wavelengths as the carrier density in the quantum well increases. This permits an approximately 30-75 nanometer tuning range in external cavity lasers by adjusting the threshold gain level. Similarly, conventional quantum well lasers have a shift in gain spectrum with carrier density that permits an approximately 10 nanometer wavelength tuning range in temperature tuned distributed feedback lasers. However, in a quantum dot laser, non-radiative interface combination and current leakage can be a particularly severe problem for quantum dot lasers having a high threshold gain because the correspondingly larger quasi-Fermi levels at high gain levels may result in a high percentage of leakage current and substantial non-radiative interface recombination. These deleterious effects can limit the ability to use a quantum dot laser structure over a large wavelength range.
What is desired are improved quantum dot structures for opto-electronics applications.
Quantum dot active region opto-electronic devices are disclosed. The quantum dot active region devices have a sequence of quantum confined energy states and a distribution in dot size that facilitates forming a continuous optical gain spectrum over an extended wavelength range. In one embodiment, the quantum dots are self-assembled quantum dots that form elongated in one direction of the growth plane. In a preferred embodiment, the mean length of the quantum dots is at least about three times their width. The distribution in dot size is preferably selected so that the inhomogenous gain broadening is at least comparable to the homogenous gain broadening. In one embodiment, the mean dot size is selected so that the optical transition energy value associated with the first excited quantum confined state is less than 30 meV greater than the optical transition energy value associated with the ground state. The quantum dot active region may be utilized in a variety of opto-electronic devices that benefit from a broad optical gain spectrum, such as tunable wavelength lasers and monolithic multi-wavelength laser arrays.